This invention relates generally to superconducting photodetectors operating at a cryogenic temperature and more particularly, to a superconducting photodetector suitable for converting a detected optical signal to an electric signal.
Heretofore known photodetectors using a superconductor operating at a cryogenic temperature include a photodetector using a Josephson device having a microbridge structure as described, for example, in IEEE Transaction on Magnetics, MAG-17, No. 1 (1981) pp. 88-91 and a photodetector using a sandwich type Josephson device utilizing the quasi-particle injection effect into a superconductor, as described, for example, in Japanese Patent Laid-Open No. 30114/1985.
The prior art technique described above detects light by utilizing the change of the electrical characteristics of the Josephson device that occurs when the light is radiated to a superconductor and the quasi-particles (electrons or holes) excited thereby in the superconductor exert influences upon thin film superconductors constituting the Josephson device.
The Josephson device described above is a two-terminal device that has only two terminals and the device itself does not have any function of signal amplification. The change of the electrical characteristics of the Josephson device occurring due to radiation of light, that is, an output signal, is smaller than a gap voltage .DELTA. of the superconductor when converted to a voltage. Accordingly, a device for amplifying the output signal of the photodetector must be disposed in order to transmit the signal to a circuit constituted by the Josephson device or a superconducting transistor. Another characterizing feature of the device utilizing the quasi-particles is that the OFF operation time of the photodetector is incomparably greater than its ON operation time and hence the circuit operation becomes complicated.
Furthermore, Physical Review, B, Vol. 18 (1978) pp. 6036-6040 discusses the photodetectors using superconductors.